The present invention is directed to an apparatus for adjustment of a mask relative to a semi-conductor wafer provided with at least one lattice structure, said mask being provided with at least one adjustment mark. The lattice structure will have a grating having different grid directions so that light striking the grating will be diffracted in different directions and the adjustment mark on the mask is opaque to the laser light so that when it is centered on the lattice structure, a lattice structure is shielded from the light and most of the diffracted light will be eliminated.
In order to be able to economically manufacture the fine structures of integrated circuits in mass production, lithographic methods are generally employed. A simple shadow projection in the light optical area is thereby increasingly replaced by more involved methods. Until a few years ago, a very simple lithographic method, namely a 1:1 shadow projection of a mask onto a wafer covered with a resist in a wavelength range of around 400 nm, i.e. in the range of soft UV radiation could be applied for generating structural details of integrated circuits on the surface of a silicon wafer. This method was distinguished by low cost, high wafer throughput and an excellent process compatibility, but the resolution of this method could not keep pace with the demands of circuit technology. Diffraction and interference effects and, in particular, the formation of standing waves due to reflections in the layer structure of this circuit cause structural imprecisions which made an application of this method impossible under production conditions given component productions smaller than 4 .mu.m. Although light-optical lithographic methods continue to be applied, these are based on the principle of imaging with the assistance of mirrors or lens optics. In the meantime, the structural region down to 1 .mu.m could be made usable with these far more precise methods known under the optical wafer stepper The limit of resolution, however, is therefore not reached. Structures down to at least 0.5 .mu.m can be governed with the optical wafer steppers under production conditions.
Given employment of a synchrotron radiation, the simple 1:1 shadow projection is possible down to the extreme sub-micron range, for example, below 0.5 .mu.m, with the assistance of x-ray lithographic without thereby having to accept limitations in the structural resolution due to the refraction, due to interference, or due to inadequate depth of field. The synchrotron radiation, for example, the relativistic radiation emission of electrons, which circulates with nearly the speed of light in a storage ring and is held on a circular orbit by means of a magnetic deflection, considerably exceeds all other x-ray sources in intensity and parallelism.
Imaging principles of x-ray lithography is extremely simple. For example, parallel x-ray emissions in the usable wavelength region between 0.1 and 2 nm impinge a mutually adjusted arrangement composed of a mask to be imaged and a silicon wafer to be exposed. A gap of typically 50 .mu.m in width, the so-called proximity spacing, is present between the mask and the wafer so that the mask and wafer do not touch one another. The exposure of the silicon wafer, similar to the exposure given an optical wafer steppers also occurs in a plurality of sub-steps in high resolution x-ray lithography, for example, in what is referred to as a "step and repeat" method. The size of the subfields, for example, the area which can be meaningfully structured with a single exposure step is defined by the processing conditioned length offset of the silicon wafer between the individual exposure steps. From today's point of view, the usable sub-field sizes even given high structural resolution lie at a few cm edge length because the image field of the objective lens, typically 1 cm.sup.2, does not act as a limitation.
Even given the great simplicity of the imaging principles of x-ray lithography, however, considerable technological problems are still to be resolved before broader application is possible. One problematical area is the adjustment of the mask and wafer relative to one another. In the structural region around 0.5 .mu.m, the required adjustment precision is at least 0.01 .mu.m. Since there are efficient optical components for actually only this wavelength region and since the energy density of the emission of lasers which can be employed for adjustment is so high that work can be carried out with extremely small adjustment mark fields, understandably, it is not x-radiation but visible light which is employed for the adjustment procedure.
In a known apparatus, the adjustment of the mask and the wafer relative to one another occurs by means of an imaging in a reflected light microscope. The simultaneous observation of the marks on both the wafer as well as on the mask produces problems with respect to the depth of field.
In another known apparatus, the adjustment of mask and wafer relative to one another occurs by means of diffraction with an interference effect. Lattice structures, which are arranged both on the mask as well as on the wafer, yield intensity modifications of common diffraction orders given dislocations of the mask and wafer relative to one another. These intensity modifications will occur due to the dislocation of the mask and wafer relative to one another; however, they cannot be separated from intensity modifications which occur due to fluctuations of the distance between the mask and wafer which intensity changes are of the same strength.
In another known apparatus, the adjustment of the mask and wafer relative to one another occurs by means of diffraction with illumination effect. Lattice structures, which are arranged on the wafer are thereby illuminated with a "Frenel" cylindrical lens which is arranged in the mask. This method is susceptible to disruptions.
In U.S. Pat. Nos. 4,211,489 and 4,422,763, whose disclosures are incorporated by reference, an apparatus is disclosed in which the adjustment of the mask and wafer relative to one another occurs by means of diffraction with an obscuring or covering of the diffraction grating. Lattice structures, which are arranged on the wafer, are thereby covered by marks which are arranged on the mask so that the intensity of the reflected, diffracted light changes dependent on the mutual position of the mask and wafer relative to one another. Given this known apparatus for the adjustment of mask and wafer relative to one another by means of diffraction with a obscuring or covering effect, the lattice structure which has four lattices having four different lattice directions, is arranged on the wafer. An adjustment mark is arranged on the mask, the outside dimensions of this adjustment mark being slightly smaller than the outside dimensions of the lattice structure on the wafer. When the laser emission irradiates an adjustment mark situated on the mask and the lattice structure on the wafer, a diffraction image of a portion of the lattice structure, which portion is not covered by the adjustment mark, will occur.